Conventionally, in order to detect a rotational position of a motor shaft, gearless elevator hoisting machines are known in which a coupling shaft is mounted to an end portion of the motor shaft, and an encoder is disposed on the coupling shaft. A plurality of screw-threaded apertures for mounting the coupling shaft on the motor shaft are formed on an end surface of the motor shaft. The coupling shaft is fixed to the motor shaft by a plurality of bolts that are screwed into the screw-threaded apertures (See Patent Literature 1).